The molecular nature of NK cell receptor(s) remains unknown in spite of considerable research efforts. Lectin-type receptors are currently among few molecules that are expressed specifically on NK cells and have the potential for transmembrane signaling and cell activation. The recent findings from several laboratories indicate the possibility that leukocyte common antigens (CD45), transmembrane protein phosphatases abundantly expressed at the surface of leukocytes in a variety of lineage- and stage- specific isoforms, may also possess the carbohydrate-binding site(s) in their extracellular ligand-binding domain(s). Inhibition studies with monoclonal antibodies and recent genetic evidence clearly indicate the central position of this molecule in NK cell cytolysis and lymphocyte actiVation. However, the critical issue important for the detailed functional studies has been the identification of physiological ligands interacting with CD45. Therefore, the evidence that certain acidic as well as neutral carbohydrates (oligosaccharides) might be among physiologically relevant ligands of CD45 merits further experimental investigation. The major objective of the proposed project would be to investigate further molecular details and possible activating function of the high- affinity oligosaccharide ligands of CD45. As a part of this study, the molecular mechanisms underlaying the carbohydrate-dependent CD45-mediated activation pathway(s) and other NK cell activation processes initiated by the high-affinity interaction of carbohydrates with CD45, will be analyzed. To accomplish these goals, the proposal addresses the following specific aims: to investigate further molecular details of the oligosaccharide-binding site of CD45 including lineage-specificity of its expression, definition of the oligosaccharide-binding domain and identification of the endogenous carbohydrate ligands from selected target cell tumor lines; to delineate the carbohydrate-mediated CD45-dependent activation pathways by biochemical methods and mutant reconstitution studies; to analyze the interaction of CD45-dependent activation by carbohydrates with other activation mechanisms important in NK cytotoxicity.